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Thick, black plumes of smoke carry for miles over northern Iraq, blotting out the afternoon sun and making it feel like night.

Huge blazes have been burning uncontrollably for more than two months near the town of Qayyarah, after retreating fighters for Islamic State of Iraq and the Levant (Isil) set fire to its oil fields to provide a cover from air strikes.

The Iraqi army managed to recapture the town from the jihadists in August, but is still struggling to repair the damage left behind.

The apocalyptic scenes are only a taste of what is to come for the troops when they push towards Iraq’s second city of Mosul, 25 miles north, in a long-awaited offensive expected to begin next week.

Thousands of Iraqi soldiers and United States special forces were yesterday seen moving towards the frontline near Qayyarah, in preparation for the country’s biggest and most complex military operation since the 2003 invasion.

From the skies, US-led coalition warplanes have been targeting Isil leaders and weapons facilities in and around the city to weaken the group before the final ground assault.

The Iraqi army has managed to recapture the town of Qayyarah from Isil terroristsCredit:
Sam Tarling/Oxfam

A defeat in Mosul would be a serious blow to Isil's legitimacy. The group has its roots in Iraq, and is where its Iraqi leader, Abu Bakr al-Baghdadi, declared the formation of the caliphate.

The estimated 8,000 jihadists in the city are expected to use everything in their arsenal to protect their last-remaining stronghold in the country, which is five times larger than any other territory it controls.

They have dug a 7ft-by-7ft trench around the perimeter of the city to be filled with burning oil once the army closes in. They are also thought to have rigged a chemical plant with explosives and plan to use some 1.5 million residents as human shields.

“Isil is panicked,” one resident, who gave only the name Ahmed to protect his identity, told The Telegraph via messaging app WhatsApp. “Since they lost Qayyarah, they have begun to tighten their security; carrying out mass arrests and raiding houses in search for weapons and illegal phones.”

Isil has done everything it can to manage the information battle within the city. Internet connections have been largely blocked in an attempt to stifle rebellion, although some manage to get access through illicit satellite equipment.

“It used to be the case that you could leave if you paid a $10,000 (£8,200) bribe, but for two months now no one at all has been allowed out,” said Ahmed. “Guards with night-vision goggles shoot at people trying to escape. If anyone makes it past them, they face the landmines that have been planted along the bridge out.”

Residents face a choice between staying in Isil-controlled areas where many have suffered extreme oppression and food shortages, or risking explosive devices and bullets to escape the fighting.

At a camp near the Isil-held town of Hawija, east of Qayyarah, families who have managed to escape have been arriving barefoot and severely dehydrated.

“Children are arriving from Hawija on the verge of death,” said Aram Shakaram, Save the Children’s deputy country director in Iraq.

“Food in the area is running out and they are hungry, thirsty and absolutely exhausted, having walked for 36 hours through mountains full of landmines and Isil patrols. Our team heard of a woman and her 17-year-old nephew who collapsed and died just a few miles away from help."

“This is just the start and we fear it is going to get much worse,” he warned.

Oxfam has distributed emergency supplies including blankets and hygiene items to families from the area that are still displacedCredit:
Sam Tarling/Oxfam

Aid agencies say the operation will likely spark the biggest humanitarian crisis in a generation, with as many as a million civilians expected to be made homeless when the fighting begins.

Already more than 100,000 people have fled to about half a dozen pop-up tent cities. The UN is scrambling to build more, but if the numbers reach the highest predictions, it could be too little too late.

"The worst case scenario in Mosul would look something like this: you would have mass expulsion of hundreds of thousands of people. You would have hundreds of thousands of people who are held as human shields," said Lise Grande, the UN humanitarian coordinator for Iraq.

“Almost every victory is accompanied by a simultaneous humanitarian crisis. It’s a lot. Sometimes it feels like too much,” she said.

Military operations along the Mosul Corridor have already displaced almost 150,000 people since March 2016Credit:
Sam Tarling/Oxfam

Preparations for the offensive have been in the making for two years, since the jihadists overran the city almost unopposed in the summer of 2014. Despite US training and sophisticated American weaponry, Iraqi troops retreated when Isil carried out its blitzkrieg across the north of the country.

The jihadists were welcomed by some of the city’s Sunni Muslim residents, who had felt politically marginalised and persecuted by the Shia-led government in Baghdad.

It is feared the offensive will stoke existing sectarian tensions.

Smoke from burning oil fields in Al Qarrayah fills the sky near the village of Imam GharbiCredit:
Sam Tarling/Oxfam

The city is home to mostly Sunnis, as well as a much smaller number of Shia Muslims, Christians and Kurds. But the offensive will be led by the Iraqi army and the Shia militias it relies on. The Kurdish Peshmerga army, which has been advancing on Mosul from the east, is also expected to take a role in helping liberate the surrounding towns and villages.

Operations to recapture other Sunni-majority cities Fallujah and Tikrit were largely spearheaded by the Shia militias and heavily coordinated by Iran's Revolutionary Guard. In the days and weeks afterwards, reports of extrajudicial torture and killings committed against the Sunni population helped harden support for Isil and deepen mistrust in other Sunni areas.

“While some residents of Mosul just want to be free from Daesh (Isil) at any cost, the majority believe it’s better to be trapped under them then freed by a Shia army,” said Mr Nujaifi.

While the rival forces share an immediate common goal of liberating Mosul, in the long term they have competing objectives.

Iraq’s government wants to reassert control over the country’s second city.

However Kurdish forces are looking to expand their semi-autonomous state, which skirts around Mosul to the north and east. They say they will keep any land they capture, which will become part of a larger Kurdistan.

The operation is complicated further by Sunni neighbour Turkey’s insistence on having some of its own troops on the frontline.

More than 2,000 Turkish soldiers have been training Sunni Muslim and Peshmerga units at Iraq's Bashiqa camp and want them involved in the assault, warning that Shia militias might reap revenge on the city’s residents.

Turkey has a historical claim to the city from the days of its Ottoman rule and wants a say in its future governance.

Baghdad has objected, however, accusing Ankara of wanting to use the Sunni tribal force as a proxy for its own interests in Mosul.

“The offensive is a great opportunity for reunification in Iraq, a chance to heal the rifts between the city and the rest of the country,” Rasha al-Aqeedi, a Mosul native and research fellow at the Al Mesbar Studies and Research Center in Dubai, told The Telegraph.

“But the involvement of all these foreign actors is problematic. Residents don’t see any legitimate forces fighting for them, which doesn’t bode well for the city’s future after Isil has left.”